1. Field of the Invention
The present invention relates generally to the field of apparatus for fiber optical communication, and more particularly, to a two-channel plastic optical fiber (POF) rotary joint.
2. Description of Related Art
typical fiber optical rotary joint consists of a fixed fiber holder and a rotatable fiber holder which are relatively rotatable each other to allow uninterrupted transmission of optical signals through the rotational interface from fibers on any one of the holders to the fibers on another holder.
There are three types of optical fiber commonly used: single mode glass fiber, multimode glass fiber and plastic optical fiber (POF). Glass fibers which support many propagation paths or transverse modes are called multimode fibers. Fibers which support only a single mode are called single mode fibers. Multimode fibers generally have a large-diameter core, and are used for short-distance communication links or for applications where high power must be transmitted. Single mode fibers are used for most communication links longer than 200 meters.
Plastic optical fibers possess a few important characters that make them favorite choices for certain short-distance fiber optical applications. Their large cores allow easy light coupling. Termination can be simplified to quick hot knife cutting or diamond turning. They are bend insensitive and do not fatigue easily.
A couple of prior inventions of two channel fiber optic rotary joint are descried in the following patents: U.S. Pat. No. 5,588,077, U.S. Pat. No. 4,842,355, and U.S. Pat. No. 4,725,116.
In U.S. Pat. No. 5,588,077, the two optic channels are arranged in-line along the same rotational axis. Isolation of one channel from the other is achieved through a novel application of gradient index rod lenses of suitable pitch. A pair of lenses is arranged adjacent each other on each side of the rotational interface and a second pair of axially aligned lenses is arranged outboard of the first pair. Gold, et al designed another two channel FORJ in U.S. Pat. No. 4,842,355. A first channel signal is delivered to an optic fiber transmitted coaxially of the stationary and rotary side, transfer across the rotational plane between the two components being accomplished by opposing centrally located optic lenses. A second channel transmitted through a second optic fiber is delivered to a lens system which converts the light into a cylinder of light coaxial with the first channel and which surrounds the optic management for the first channel. Second channel thus are converted into coaxial hollow cylinders of light. These cylinders of light are transmitted between facing lens systems in the rotary and stationary sides of the apparatus. But the facing lens systems are very difficult to be fabricated.
Spencer, et al shows in U.S. Pat. No. 4,725,116 a two-channel and multi-channel FORJ. Within the joint reflecting surface are used to redirect off-axis optic signals onto the joint axis, with relative rotation occurring while the signals are on-axis. A rotating member for each channel has a reflecting surface for reflecting the on-axis signal portion off-axis to a receptor fiber. Alignment between the rotating member and the receptor fiber, as well as drive for the rotating member, is provided by a pair of magnets of opposite polarity, one being secured to the rotating member and the other being secured to the rotor. But it could be very difficult for the magnetic interaction to accurately ensure the synchronous rotation of the rotor and the rotating member. The size of the magnetic element and the adjustment of the reflecting surface also increase the size of the invented embodiment.
Unfortunately, almost all the prior arts of fiber optical rotary joints are hardly used for plastic fiber applications.